Digital cameras that store the optical image of a subject in digital form are now in actual use. An advantage of image data captured by a digital camera is that the data is easy to utilize and manipulate. However, this means that the image data can also be altered with ease. A problem which arises as a consequence is that the trustworthiness of a photograph taken by a digital camera is inferior to that of an emulsion photograph and lacks the ability to serve as evidence. For this reason, a digital camera system having a function for adding a digital signature to captured image data has been proposed in recent years. For example, see the specifications of U.S. Pat. No. 5,499,294 and Japanese Patent Application Laid-Open No. 9-200730.
However, since public-key encryption schemes such as RSA encryption ordinarily used in generating digital signatures require an exponentiation operation or a remainder operation, high-speed processing is difficult and a processing time that is several hundred to several thousand times longer than that needed for a common-key encryption scheme such as DES is required. Consequently, with the limited computational resources available in a digital camera, generating a digital signature using public-key encryption is very difficult.
Accordingly, there has been proposed a technique (e.g., see the specification of Japanese Patent Application Laid-Open No. 2002-244924) through which a digital signature is applied using a storage medium incorporating a microprocessor, e.g., an IC card, thereby enabling a digital signature to be applied to generated image data without greatly enhancing the capabilities of computational resources available to the digital camera.
However, the arrangement set forth in the specification of Japanese Patent Application Laid-Open No. 2002-244924 is such that the memory in an IC card is furnished with only one secret key with regard to a camera-specific ID. If a special user is provided with a secret key different from that of the general user, therefore, then the IC card having the secret key for the special user must be re-fabricated. The problem which arises is an increase in the manufacturing cost of the IC card for the special user.